p53 is the most commonly mutated tumor suppressor in human cancers and plays critical roles in cell cycle arrest, apoptosis, differentiation and DNA repair. While recent studies also link p53 to aging, the exact roles of p53 in aging remains unclear because premature aging is only observed in some but not all mouse models that have increased p53 activities. In responses to various stresses, p53 protein levels are upregulated and its transcription activities activated. Posttranslational modifications of p53, including phosphorylation and acetylation, are thought to play important roles in regulating p53 responses to genotoxic and oncogenic stresses. To address the physiological roles of p53 posttranslational modifications in regulating its stability and activity, we have employed mouse knock-in technology to introduce missense mutations into the phosphorylation or acetylation sites of the endogenous p53 gene in embryonic stem (ES) cells and mice. These genetic studies have provided important and sometimes surprising insights into the complex regulation of p53. With these initial successes, I propose to continue to employ mouse genetics to investigate the roles of various phosphorylation and acetylation events in regulating p53 activity, and more importantly, in p53-dependent tumor suppression and aging in vivo.